Airborne electromagnetic surveying has been a widely used method for obtaining geophysical information. Electromagnetic surveying was originally designed for the exploration of conductive ore bodies buried in resistive bedrock, but at the present time it is also used extensively in general geological mapping, in hydrogeology, in environmental investigations, etc. Known methods utilize electromagnetic conductivity techniques to measure the apparent conductivity of the earth by applying an artificial alternating magnetic field. In essence, these techniques employ a transmitter to radiate a primary electromagnetic field, which in turn induces eddy currents in underground conductors. These eddy currents induce a secondary electromagnetic field that is then observed by an electromagnetic sensor (such as a receiver coil). This data is then used to compute geophysical information in a manner that is known.
The two basic types of electromagnetic techniques are frequency domain electromagnetic (FDEM) surveying and time domain electromagnetic (TDEM) surveying. FDEM measures the electrical response of the underground conductors at different frequencies to record the variations of conductivity with depth. TDEM, on the other hand, measures the electrical response of the underground conductors to a periodic magnetic pulse. For either method, the secondary fields are measured and used for mapping and geological interpretation in a manner that is known.
Although these electromagnetic techniques encompass both ground and airborne applications, airborne systems are preferred if the speed of the surveying is important.
The common technical means to generate magnetic field pulses is a known transmitter generally consisting of a loop of wire or a multi-turn coil connected to the output of a known electrical current generator or transmitter driver. The typical size of a transmitter coil is up to a few meters in diameter for an airborne device and up to hundreds of meters for ground systems. Generally, the bigger the transmitter coil diameter the stronger its magnetic moment, which then results in deeper and more accurate investigations. An additional multi-turn coil or an x-y-z coil system usually serves as a receiver or sensor for the secondary electromagnetic field. Magnetometers are also applicable for this purpose. In contemporary systems, received signals are digitised by a known analog to digital converter (ADC) and processed and stored by computer.
In one type of electromagnetic bird, the transmitter loop is rigidly mounted inside the bird body along its perimeter and the electromagnetic sensor is mounted inside the bird body in its center. This sensor receives both the primary electromagnetic field of the transmitter loop and secondary electromagnetic fields of eddy currents induced in the underground conductors. The primary signal component, being generally constant, can be thus subtracted from the received signal using known compensating coils or electronic circuits.
One significant technical problem for these airborne systems of this type is that any mechanical deformations of the transmitter loop can change its magnetic field, and therefore induce signals in the electromagnetic sensor. It is virtually impossible to distinguish such changes from electromagnetic anomalies received from the underground conductors. For this reason, it is important to minimize the bird deformations during the survey flight. Another difficulty encountered with such systems is mechanical management in start/landing and in flight manoeuvres.
An example of airborne electromagnetic birds technology is that used in the AEROTEM™ branded solution of Aeroquest Ltd. The suspension system consists of a tow cable and three ropes, which are attached to the rigid and relatively heavy structure of EM bird at three different points. The primary disadvantage of this method of suspension is the deformation that occurs between the three suspension points in the case of vertical accelerations of the helicopter. As discussed, such deformations can serve to significantly distort the electromagnetic signal during measurements. Such three point suspension systems also may limit the bird size and weight because long distances between suspension points can cause instability and, in the worst scenario, result in breakage of the bird structure.
There are other suspension systems consisting of the tow cable and more than three ropes attached to the electromagnetic EM bird having some flexibility in the suspension points. An example of this configuration is described in U.S. Patent Application No. 20050001622. The primary disadvantage of this type of suspension system is that structural deformation can occur as a result in the change of rope length and flexing in oncoming wind conditions, causing deformations of the bird structure and consequently potentially comprising the integrity of the survey data.
Other common configurations for suspension systems exist, including one- and two-point suspensions. These are typically used only for small towed birds, such as for FDEM or electromagnetic birds containing sensors only. An example of these suspension techniques is found in the GEOTEM™ and MEGATEM™ TDEM systems (Fugro Airborne Surveys Ltd.), or the helicopter towed system manufactured by T.H.E.M Geophysics Inc. These one- or two-point suspension configurations possess the same shortcomings as described for the other suspension methods, namely that they do not provide adequate mechanical stability for larger birds.
On the basis of the foregoing, there is a need for a net suspension apparatus that provides a uniform distribution of tension forces from the aircraft to the electromagnetic bird body, thus minimizing possible deformations and optimizing the quality of the surveying data.